1. Understanding Load Type
The first step in any motor selection process is characterising the driven load. This defines the torque and speed requirements across the full operating range, which in turn determines the required motor type, power rating, pole count, and whether a VFD or gearbox is needed. There are three fundamental load categories.
Torque remains approximately constant regardless of speed. The required motor power therefore increases proportionally with speed. Examples: conveyors, positive-displacement pumps, compressors, mixers, and hoists. The motor must be sized to provide sufficient torque at the lowest operating speed, including during starting under full load.
Torque increases with the square of speed; power increases with the cube of speed. Examples: centrifugal pumps, fans, and blowers. At 50 percent of rated speed, the load only requires 25 percent of rated torque and 12.5 percent of rated power. This characteristic makes variable torque loads ideal for VFD speed control, where large energy savings are achievable at reduced flow demands.
Power remains constant while torque decreases as speed increases. Examples: metal cutting machine tools (spindle drives), winding machines, and some rolling mill drives. Motor selection for constant power loads must ensure the motor can provide full rated power at the highest required speed and sufficient torque for cutting or winding force at the lowest speed in the operating range.
Identifying the load type first prevents the most common motor oversizing error: selecting a motor sized for starting torque on a centrifugal load that only requires 12.5 percent of rated torque at half speed. An oversized motor runs at low power factor and reduced efficiency across most of its operating range, increasing both energy costs and reactive power charges.
2. Motor Power Calculation
Once the load torque-speed characteristic is known, the required motor shaft power can be calculated. The required power must be sufficient for the worst-case operating condition — typically maximum load at maximum speed — with a service factor applied for starting conditions and load variations.
| Application | Starting Service Factor | Running Service Factor | Notes |
|---|---|---|---|
| Centrifugal pump / fan | 1.0 | 1.0–1.15 | Low starting torque; size for max operating point |
| Belt conveyor (loaded start) | 1.5 | 1.15–1.25 | High inertia or loaded start requires starting torque margin |
| Screw compressor | 1.5–2.0 | 1.25 | Unloaded start possible; use soft starter or star-delta |
| Agitator / mixer | 2.0–2.5 | 1.25–1.5 | High viscosity batch start is the worst-case condition |
| Hoist / crane | 2.0–3.0 | 1.5 | Starting under full suspended load; use brake motor |
After calculating the required motor power, round up to the next standard IEC power rating in the series (0.18, 0.25, 0.37, 0.55, 0.75, 1.1, 1.5, 2.2, 3.0, 4.0, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90, 110, 132, 160, 200 kW). Avoid selecting a motor more than one standard size above the calculated requirement, as this increases capital cost and reduces part-load efficiency and power factor.
3. Speed, Poles and Gearing
Motor speed in revolutions per minute is determined by the number of magnetic poles and the supply frequency. At 50 Hz, the four standard pole configurations produce these synchronous speeds and approximate full-load speeds:
When the required output speed does not match any standard motor speed, a gearbox is used to step down (or occasionally step up) between motor and load. The gear ratio required is simply the motor speed divided by the required output speed. For example, a conveyor requiring 28 rpm from a 4-pole motor at 1,450 rpm needs a gear ratio of approximately 52:1. This would typically be achieved with a worm gearbox for modest torques or a helical gearbox for continuous heavy duty at high power.
Selection tip: When the required output speed is between 50 and 300 rpm, compare the total cost of a 4-pole motor with a two-stage gearbox against a 6-pole or 8-pole motor with a single-stage gearbox. The latter option often reduces drivetrain complexity, maintenance points, and total installed cost for agitator and conveyor applications. Korea Ever-Power’s gearmotor combinations cover all four pole configurations matched with worm, helical, and bevel-helical reducers.
4. Duty Cycle and Service Factor
IEC 60034-1 defines nine standard duty types (S1 through S9) that describe how a motor’s load and rest periods alternate over time. The duty cycle determines the thermal loading on the motor and directly affects the required power rating.
Motor runs continuously at constant load for long enough to reach thermal equilibrium. The most common duty type — pumps, fans, compressors, conveyors. Motor is rated for continuous operation at the nameplate power.
Motor runs at constant load for a short defined period (10, 30, 60, or 90 minutes), then stops long enough for full cooling to ambient. Valve and sluice gate actuators, intermittent machine tools. Short-time duty motors can be rated above S1 continuous power for their defined run period.
Motor alternates between on-load and rest periods in a fixed cycle, and never reaches thermal equilibrium in either phase. Cyclic duty factor (CDF) is the ratio of on time to total cycle time. Packaging machines, presses, and elevator drives often operate on S3 duty with CDF of 25 to 60 percent.
S4 includes thermal effect of starting. S5 includes braking cycles. S6 covers continuous operation with intermittent overloads. S7 and S8 cover motor braking and speed-changing duties. These duty types are relevant for crane, hoist, and rolling mill applications where frequent acceleration-deceleration cycles impose additional thermal stress on the motor winding.
5. Operating Environment and IP Class
The installation environment determines the required IP ingress protection rating, insulation class, and in some cases the motor construction material. A motor installed in a clean, dry indoor environment can use IP44. The same motor in a chemical plant outdoor area subject to rain and dust requires IP55 or IP65. Food processing lines subject to daily high-pressure caustic washdown require IP69K with stainless steel construction.
For classified hazardous areas containing flammable gases or combustible dust, standard motors are not permitted regardless of IP rating. Certified explosion-proof motors (Ex d IIB T4 for Zone 1 gas areas, or Ex t IIIB for Zone 21 dust areas) are mandatory. Korea Ever-Power’s YB2 series explosion-proof motors cover Zone 1 and Zone 2 gas areas from 0.55 to 200 kW in all four pole configurations.
| Environment | Min. IP |
|---|---|
| Clean indoor, dry | IP44 |
| General industrial, dusty | IP54 |
| Outdoor or water jets | IP55 |
| Washdown areas | IP65 |
| Food / pharma washdown | IP69K |
| Hazardous gas area (Zone 1) | Ex d IIB |
6. Motor Type Selection Matrix
Once load type, power, speed, duty cycle, and environment are defined, the motor type can be selected. This matrix matches the most common application requirements to the appropriate motor series from Korea Ever-Power.
| Requirement | Korea Ever-Power Series | Key Feature |
|---|---|---|
| Standard S1 continuous duty, clean indoor | Y2 Series | IE3, IP54, 0.18–200 kW, all 4 pole configurations |
| Variable speed with VFD | YVF2 Series | IC416 external blower, Class H, PTC thermistors, 0.75–200 kW |
| Two fixed speeds needed (no VFD) | YD Series | Dahlander / separate winding, 4/2P through 12/6P pole combos |
| Load-holding / positioning duty | Y2EJ Series | Integral spring-applied DC brake, stop time < 0.1–0.4 s |
| Hazardous area Zone 1 / Zone 2 | YB2 Series | Ex d IIB T4, IP55, 0.55–200 kW, IECEx / ATEX certified |
| IP69K daily washdown (food / pharma) | BXG Series | 316L stainless, IP69K, FDA seals, Ra 0.8 um, Class H |
| Small power (< 1 kW), aluminium frame | YS Series | 25 W–750 W, aluminium frame, IP44, capacitor-start option |
Full product specifications, model tables, and application guides for each series are in the three-phase motor product section.
7. Efficiency Class and Lifecycle Cost
For most industrial motors, the energy cost over a 10-year operational life far exceeds the initial purchase price. A 22 kW motor purchased for 600 USD will consume approximately 480,000 kWh over 10 years at full load (4,800 hours per year). At 0.12 USD/kWh, this is 57,600 USD in electricity costs — 96 times the motor purchase price. An efficiency improvement from IE2 (90.9%) to IE3 (91.6%) at 22 kW saves approximately 410 kWh per year per motor, or 4,100 kWh and 492 USD over 10 years at 0.12 USD/kWh.
For facilities with large motor populations, the accumulated energy saving from upgrading from IE2 to IE3 across 50 to 100 motors can represent significant annual operating cost reduction. The EU Regulation 2019/1781 mandates IE3 as the minimum efficiency class for motors from 0.75 to 200 kW in the European market, and many other markets are adopting equivalent regulations. Specifying IE3 as a minimum for all new motor purchases is both regulatory compliance and sound operational economics.
8. Frequently Asked Questions
Edited by Cxm